Currently, the transmission speed of high-speed optical communication links ranges from a traditional range of 1.25 Gbps-10 Gbps to up to 25 Gbps for single-channel links. An increase in the transmission rate requires increased bandwidth of transmission links. A bandwidth of 7 GHz is required to transmit 10 Gbps signals, and a bandwidth of 21 GHz is required to transmit 25 Gbps signals. A plurality of carriers exist in high-speed links of optical modules. The carriers normally include:
a printed circuit board (PCB), which is a carrier used for surface-mount device (SMD) components, which can be linked with a system, and is low cost;
a flexible printed circuit (FPC), which is similar to a PCB and has advantages of being able to interconnect two hard carriers and absorb space tolerance;
an airtight ceramic box, which is used in device packaging that has requirements for high airtightness, such as lasers, thermoelectric coolers (TEC), photo detectors (PD), etc.; and
a ceramic substrate, which is used for optoelectronic devices and has good heat dissipation properties and high processing precision.
In general, a PCB and an FPC, and an FPC and a box may be soldered using a tin-soldering process; and an optoelectronic device and a ceramic substrate may be soldered using a gold-tin soldering process. In order to establish connections between the ceramic substrate and transmission lines (PCB, FPC, or Ceramic Box) over an entire link, two types of carriers normally undergo a gold wire bonding process. The relatively simple operation, easy automation control, and high reliability make a gold wire bonding process a convenient option in the packaging process. The equivalent model for gold wiring is electrical induction; direct induction at high frequencies is associated with severe bandwidth attenuation. Consequently, the bandwidth of the entire link is directly affected by the interconnect bandwidth between the ceramic substrate and the transmission lines.
In terms of single-mode transmissions on the market, distributed feedback (DFB) lasers are available at low cost and are widely employed in long-distance transmissions. While DFB lasers usually employ current modulation, they have a relatively small inherent internal resistance that is usually 10 ohms. However, the impedance of transmission line is usually designed as differential 50 ohm, which can produce a reflection that affects signal quality. Moreover, the drive current of a DFB laser is relatively large, and, as temperature increases, the corresponding drive current must be increased to meet transmission requirements. Since lasers have a relatively high power consumption density, a good method for heat dissipation is necessary for lasers to meet optimum working conditions.
As shown in FIG. 1, a substrate packaging structure of an optical module includes a first substrate 10′ and a second substrate 20′ that are electrically connected. Electrodes 11′ are arranged on the first substrate 10′ and electrically connected to the first substrate 10′. Transmission lines 21′ and a metal layer 22′ for transmitting signals are arranged on the second substrate 20′. The first substrate 10′ and the transmission lines 21′ on the second substrate 20′ are electrically connected through connection lines 31′ in order to transmit laser signals.
In the packaging structure of FIG. 1, the first substrate 10′ is made of aluminum nitride ceramic, is gold plated, and has a high thermal conductivity coefficient. The electrodes 11′ are soldered onto the first substrate 10′ by a gold-tin soldering process. Then the entire assembly is soldered onto a heat sink, and finally connected to an outer case of the module for heat dissipation. In terms of the electrical performance of a laser, generally the cathode of a laser is soldered onto the traces of the gold-plated first substrate 10′ through a gold-tin soldering process, and the anode of the laser is soldered onto another gold-plated surface of the first substrate 10′ through a gold wire bonding process. Then the first substrate 10′ is electrically connected to the second substrate 20′ (PCB or FPC) through gold wires. The result from a time domain reflectometry (TDR) simulation is shown in the graph of FIG. 2. In the graph of FIG. 2, the curve represents impedance versus time, and the time is related to a position. The peak of the curve shows that the impedance of the packaging structure is 59.5 ohm.